1. Field of the Invention
This invention relates to capstan and head servo systems for video tape recording and reproducing apparatus and more particularly to such systems having the capability of permitting material to be added or replaced without any significant errors caused by the transition from material previously recorded to the added video signal on the tape.
2. The Prior Art
When information is recorded on magnetic tape, it is frequently desirable to edit the material, for example to replace part of the information by different information, or to add new material. The process of adding new information at a certain point in the recording is called an assemble edit operation. A typical procedure is for the editor to run the tape through a video tape machine that is capable of both recording and playing back video signals. Setting the machine to play back the recorded information, the editor watches a television monitor until the scene reaches a point at which the new material is to be added. The machine is stopped at that point and the tape is rewound part of the way. Then the tape is started again and, when the tape reaches the scene previously noted by the editor, the operation is suddenly switched to cause the new material to be recorded, starting at the designated point. The original operation just described, to the point at which the tape is stopped, is a normal playback operation. The operation after the tape was rewound is referred to as an edit playback operation up to the point that the new material starts being recorded. From there on to the end of the recording of the new material, the operation is referred to as an edit record operation.
A video tape recorder transport mechanism requires means to drive a capstan to pull the tape along its operating path at a predetermined speed. Part of the path is a helical section at least part way around a drum that has a gap that coincides with the perimeter of rotational movement of, usually, two diametrically opposed transducers that can be used for recording or for playback operation. The angular position of the support mechanism for these transducers, or heads, must be precisely and constantly correlated with the movement of the tape and the television signal to be recorded.
Because the tape follows a helical path and the heads follow a circular path, the video information is recorded on the tape in a series of slant tracks that are at an angle to the longitudinal direction of the tape. Typically, each slant track records all of the information on one television field and two adjacent slant tracks record all of the information in one television frame. The correlation between the position of the heads and the television signal is such that, as each head starts to record a new track, the first information recorded is the information during the vertical blanking interval.
A typical video tape transport mechanism includes a motor to drive the heads at the proper speed and a separate capstan motor. In order to correlate the position of the heads with the television signal, the shaft on which the heads are mounted has a magnet that moves past a fixed coil as the shaft rotates. Each time the magnet does so, it produces a pulse in the coil when the heads are in a specific angular position. This pulse is compared with a signal derived from the vertical synchronizing signal of the composite television signal, and the mechanism is adjusted so that when the requirements of the comparison circuit are satisfied, the heads will be in position to begin a new track at the proper time with respect to the television signal.
The capstan motor may be a direct current motor which can run at any speed. A speed control circuit may be provided in the form of a magnet attached to rotate with the capstan motor shaft, but this magnet is provided with a number of poles so that it generates a relatively high frequency signal in a coil located close enough to be intercepted by the magnetic fields of the individual poles. A frequency detector circuit produces a direct voltage, the magnitude of which is a function of the frequency of the pulses induced in the capstan's speed control coil. This direct voltage is used to control the operating current of the motor to cause the motor to operate at the desired speed or very close to it. The speed of the capstan motor is controlled even more precisely by means of a servo circuit utilizing another magnet on the shaft of the capstan to generate a signal in another coil. The latter signal is compared with a vertical synchronizing signal or a signal derived from the head shaft but operated synchronously with the vertical synchronizing signal.
The interconnections between the motors and the servo circuits therefor have taken several forms in tape machines constructed heretofore. Such interconnections differ during various modes of operation, i.e., the recording mode, the playback mode and the edit record mode. During the initial recording, it is necessary that the rotation of the head shaft and therefore, of the capstan, be controlled by the external video signal being recorded. In order to control the subsequent playback operation, a control signal is simultaneously recorded along one edge of the tape by means of a fixed head. This control signal is recorded as a series of pulses, each of which corresponds with one of the pulses derived from the vertical synchronizing signal. During normal playback operation there is no external signal and thus the video tape machine must be controlled by signals recorded upon the tape. The head motor, being a synchronous motor, operates at a specific speed, but its phasing is controlled by the pulses recorded along the edge of the tape.
In a video machine arranged for editing, there is an important difference between the normal playback operation and the edit playback operation. In the edit playback operation, even though the machine is reproducing signals recording on the tape passing through it, the phase control of the head is governed by signals derived from the composite video signal that is to be recorded once the proper point is reached. This makes it possible to shift instantaneously from the edit playback mode to the edit record mode at the edit point, and the slant tracks of the newly recorded material will fall into exactly the same positions as the slant tracks of the material being deleted in the section of the tape that follows edit point.
However, in a conventional video tape recording machine, since the reference signal of the capstan phase servo circuit is supplied from different signal sources during the edit playback and the edit record modes, an out-of-phase condition may be induced in the capstan circuit at the assemble point.
Accordingly, it is an object of the present invention to provide an improved system for effecting proper phase lock synchronizism between the rotational phase and speed of the head and the rotational phase of the capstan for normal video recording and reproduction.
It is another object of the invention to provide a system that permits electronic editing of a new video signal in place of an existing recorded video signal, such editing to take place in a controlled manner and without any significant time base error.